Aluminum-silicon alloys have been used for transistor gates and interconnect lines within integrated circuits for years. The primary advantages of the alloy are that it is inexpensive, easy to etch and a relatively good conductor. The reason that silicon is alloyed with the aluminum is that during glassing encapsulation operations, temperatures typically range between 400.degree. to 560.degree. C. Within this temperature range, aluminum, if not alloyed with silicon in order to achieve the solubility limit at the maximum process temperature, tends to diffuse into adjacent silicon regions of the circuit, destroying the semiconductor status of the silicon material so invaded. Aluminum-silicon alloys having a silicon content of 0.5 to 2 percent by weight are commonly used.
Although saturating aluminum with silicon solves the diffusion problem, another problem is created which particularly manifests itself as device dimensions are shrunk. High temperatures inherent in contemporary manufacturing processes causes the silicon within the saturated aluminum-silicon alloy to precipitate, especially at junctions between the alloy and silicon regions. In fact, epitaxial growth of monocrystalline silicon can actually occur at such junction regions, with the alloy contributing the silicon as growth progresses. As silicon precipitates or grows epitaxially in junction regions where electrical connections have been made, the quality of the connections decreases, often to the point of total failure. FIG. 1 is a 1,980.times. photomicrograph of silicon precipitate crystals that remain on a silicon substrate following the dissolution of an adjacent 1 percent silicon-aluminum alloy layer after the substrate and alloy layer were maintained at 400.degree. C. for one hour. It will be noted that a large percentage of the crystals are of relatively uniform size; the remainder are considerably larger. FIG. 2 is a 15,000.times. view of some of the precipitate crystals shown in FIG. 1.
Semiconductor manufacturers have solved the problem of silicon precipitates by depositing thin barrier layers on top of the contact region silicon. These barrier layers prevent the migration of aluminum atoms, which eliminates the need to incorporate silicon in the aluminum alloy used for interconnect lines. The silicon precipitate problem is thus solved. However, the deposition of the barrier layer adds times, complexity and cost to the fabrication process.
Hot and cold temperature cycling inherent in the manufacturing process can also create stresses that cause aluminum-silicon alloys to crack. Cracks in interconnect lines are hardly acceptable, as cracks may result in open circuits.
In addition, aluminum interconnect lines, even when alloyed with silicon, are likely to fail in use due to current-induced mass transport of aluminum atoms from one part of the interconnect line to another. The underlying physical phenomenon which induces such failure is generally termed "electromigration". Electromigration occurs in a current-carrying conductive material maintained at elevated temperature as a result of the combined effects of direct momentum exchange from the moving electrons and the influence of the applied electric field. Failures attributable to electromigration generally result either from the complete removal of a portion of an interconnect line (an open circuit) or the buildup of material between previously-isolated conductive paths (a short circuit). However, the protective ability of an overlying protective layer, such as an encapsulating insulating layer, may also be impaired or fractured as a result of material removal or build-up. Such an impairment or fracturization of the protective layer may then subject the circuitry to deleterious environmental conditions (e.g. humidity) which might hasten component failure.
The problems of stress-cracking and electromigration heretofore described may be largely alleviated by the use of copper-doped aluminum interconnect lines. Irving Ames and his coworkers at IBM Corporation discovered that a percentage of copper approximately in the range of about 0.1 to 10 percent by weight composition was sufficient to virtually eliminate the electromigration effect in aluminum-alloy interconnect lines, particularly if the alloy was annealed at a temperature within the range of 250.degree. to 560.degree. C. This discovery is described in U.S. Pat. No. 3,879,840. It was subsequently determined that copper-doped aluminum interconnect lines were less likely to undergo stress cracking under temperature cycling. Although attempts have been made to reduce electromigration susceptibility in aluminum-silicon alloy interconnect lines by incorporating a small percentage of titanium in the alloy, the results were not uniformly successful. For this reason, aluminum-silicon-titanium alloys are not commonly used as interconnect lines within integrated circuits.
Although the doping of aluminum interconnect lines with copper ameliorates the problems of stress cracking and electromigration, it exacerbates the problem of silicon precipitation at silicon-to-metal junctions. This fact is demonstrated by FIG. 3, which is a 2,000.times. photomicrograph of silicon precipitate crystals that remain on a silicon substrate following the dissolution of an adjacent 1 percent silicon-0.5 percent copper-aluminum alloy layer after the substrate and alloy layer were maintained at 400.degree. C. for one hour (the same high-temperature step experienced by the substrate and alloy layer of FIGS. 1 and 2). It will be noted that the ratio of large to small crystals has increased substantially. Therefore, the possibility of a metal to silicon junction failure is substantially increased. FIG. 4 is a 15,000.times. view of some of the precipitate crystals shown in FIG. 3. FIG. 5 is a photomicrograph of the cross-section of a DRAM cell. The photo is particulary noteworthy because it shows the junction 51 between a doped silicon substrate 52 and an aluminum-copper-silicon alloy bit line 53, the junction 51 having been rendered useless by the precipitation of silicon crystals. The micrograph of FIG. 6 shows a similar bit-line connection region following the removal of the metal with an acid bath etch. Three silicon crystals 61A, 61B and 61C remain on the substrate 62 in what was the metal-to-substrate contact region, with the largest crystal 61A displaying the rather obvious shape of epitaxially-grown, mono-crystalline silicon. The precipitation problem associated with aluminum-copper-silicon alloys at alloy-to-silicon junctions has heretofore prevented the scaling down of such junction regions, since junction surface area was required to be of sufficient size to cope with any silicon precipitation that was likely to occur at the contact site.
What is needed is an improved aluminum alloy for interconnect lines which possesses both acceptable resistance to electromigration, and improved silicon precipitation characteristics.